Wireless local area networks (WLANs) have become ubiquitous. The original WLAN standard, IEEE 802.11, was designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. Subsequently, IEEE working groups defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
A recent amendment to the standard, IEEE 802.11n-2009, is designed to increase in the maximum data rate from 54 Mbit/s to 600 Mbit/s, by using multiple spatial streams. This amendment, whose full title is IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 5: Enhancements for Higher Throughput (Oct. 29, 2009), is incorporated herein by reference. IEEE 802.11n mandates multiple-input multiple-output (MIMO) technology, in which multiple transmit antennas and multiple receive antennas are used to simultaneously transmit multiple spatial streams.
The maximal data rate of 600 Mbit/s under the 802.11n standard is achieved by using four such streams over respective 40 MHz-wide channels, with 64-QAM modulation and a ⅚ forward error correction (FEC) coding rate. This maximal level requires that both transmitter and receiver use at least four antennas, with excellent channel conditions. Since these operating conditions are often not achievable, the standard defines over thirty-two different modulation and coding schemes (MCSs), each with its own maximal data rate. Each scheme is denoted by a MCS index (between 0 and 31) and is characterized by a certain number of spatial streams (between one and four), a modulation type (BPSK, QPSK, 16-QAM or 64-QAM), and a coding rate (between ½ and ⅚). When two stations with 802.11n capability communicate, they choose the MCS index to use between them by a trial-and-error process. Each station makes an autonomous decision regarding the MCS to use in sending packets to its counterpart.
In a classical 802.11 WLAN, each access point operates independently of the other access points and has its own, unique basic service set identification (BSSID). On the other hand, U.S. Pat. No. 7,797,016, whose disclosure is incorporated herein by reference, describes a WLAN with central management of access points, in which the access points all share a common BSSID. Upon receiving an uplink signal, transmitted over the WLAN by a mobile station, at one or more of the access points, the access points forward uplink management and broadcast messages over a wired LAN to an access point manager. The manager selects one of the access points to respond to the mobile station. Problems of overlapping coverage areas and collisions are thus resolved, typically in favor of the access point that has the strongest reception of uplink signals from the mobile station in question.